Semiconductor switches, such as MOSFETs, are commonly used as “high-side” switches. In an example of such an arrangement the source of the N-channel MOSFET is connected to the high voltage side of a load and a drain of the N-channel MOSFET is connected to a voltage source and vice versa for P channel MOSFET's. In practice the load is typically connected between the N-channel MOSFET and a local ground either directly or via a low side switch. Therefore when the MOSFET high side switch is in a high impedance state the voltage occurring at either side of the load is effectively the ground voltage. This is recognised as a generally desirable feature. The load is typically supplied from a power supply which may be at a relatively high voltage, for example several hundred volts. In contrast, a control system which is responsible for instructing the high side switch to be turned on or off will generally operate at a significantly lower voltage of generally only a few volts or at most a few tens of volts. It is therefore necessary to provide an interface between the low voltage control system and the MOSFET switch. This interface can be regarded as an electronic switch driver circuit which generally requires an independent floating power supply.
The driver circuit can be used to provide enhanced protection against fault conditions. It is, for example, known to include a current measuring capability within the driver circuit and to use the driver circuit to automatically initiate opening of the semiconductor switch (that is placing the semiconductor switch in a high impedance state) in the event that excess current flow is detected. This safety feature is, in a well designed system, only invoked in response to a fault condition. Consequently this safety feature may not be used for a considerable period of time. It is therefore possible that a fault might develop in relation to this safety feature within the driver circuit, and that such a fault may remain dormant, that is not noticed, for a considerable period of time.